1. Field of the Invention
The present invention relates to a technique of preventing foreign matter from adhering to the surface of an optical member arranged on or near a focal plane, such as a solid-state image sensor, an optical filter, or a lens, in an image capture apparatus.
2. Description of the Related Art
A digital camera takes an image by exposing a solid-state image sensor that converts light energy into electrical signals. In such a digital camera, unlike a film camera, an image is taken using a solid-state image sensor. Accordingly, if foreign matter happens to adhere to an optical element disposed in front of the solid-state image sensor, the foreign matter may remain unremoved, and a portion to which the foreign matter have adhered may be captured as shading in images that are taken thereafter. Possible causes of the occurrence of such foreign matter include the following, especially in an interchangeable lens-type single-lens reflex camera.
(A1) In a camera having a focal plane shutter, shutter blades of the focal plane shutter rub against each other when the shutter works, and paint on the shutter blade may be peeled off and may remain as foreign matter in the camera.
(A2) In an interchangeable lens-type camera, when a mount cap, which is used to protect a mount of the camera while an interchangeable lens is not attached to the camera, is attached to the mount, the mount cap may be scraped due to rubbing between the mount cap and the mount, and the scraped pieces of the mount cap may occur as foreign matter.
(A3) When interchangeable lenses are exchanged, some foreign matter may enter the inside of the camera from the outside.
In at least some of the causes of foreign matter remaining in the inside of the camera as discussed above, especially in the above causes (A1) and (A2), foreign matter that occurs inside the camera occurs due to exfoliation. Therefore, the foreign matter may be charged with electricity.
Further, possible causes of foreign matter adhering to an optical element include the following.
(B1) When an optical element is charged with electricity as discussed below, an electric field occurs between the optical element and the shutter blade, and electrostatic force works on the charged foreign matter. Therefore, the foreign matter is attracted towards the optical element and adheres thereto.
(B2) Due to airflow or vibration inside the camera, foreign matter adheres to the optical element.
Herein, the reasons for electrification of the optical element that leads to the above cause (B1) are discussed. The reasons for the electrification of the optical element include mainly two reasons as described below.
(C1) In a camera during photographing, when a main mirror is flipped up, a member supporting the main mirror comes into contact with a stopper portion on the top of a mirror box. Therefore, contact charging occurs. In addition, friction charging occurs by friction of the shutter blades in a shutter operation. By these causes, potential changes occur in the shutter blades. Therefore, electricity is induced to the optical element, which is then charged with electricity.
(C2) Crystal and lithium niobate used as a low-pass filter of the optical element are materials with pyroelectricity. Pyroelectricity is the electrical potential created in a particular crystal, in which when the crystal is polarized as a result of a change in temperature, or when a piezoelectric effect occurs due to deformation by temperature changes, the crystal material is charged with electricity. Therefore, when the pyroelectric low-pass filter is charged with electricity, the whole optical element is charged with electricity.
The reason (C1) among the reasons for such electrification of the optical element is also applicable to a solid-state image sensor located near a focal plane shutter in the case of a camera in which no optical element is disposed in front of the solid-state image sensor. Thus, the reason (C1) leads to adhesion of foreign matter onto the surface of the solid-state image sensor.
A method of preventing foreign matter from adhering to the surface of an optical element or a solid-state image sensor including a charged member disposed around an optical element is discussed in Japanese Patent Application Laid-Open No. 2003-37756. In this configuration, the charged member disposed around the optical element has a larger potential than that of the optical element. Therefore, charged foreign matter is attracted to the charged member and is, thus, prevented from adhering to the optical element. However, electric consumption increases due to driving of the charged member, and a space for the charged component is required. Therefore, a problem arises in that the camera size cannot be reduced.
In addition, a charging prevention process is discussed in Japanese Patent Application Laid-Open No. 5-307403, in which conductive fine particles are dispersed in a reflection reducing film on the surface of an optical element. In this process, a highly conductive layer is arranged in an optical element to thereby prevent electrification of the optical element, so that adhesion of foreign matter onto the optical element due to electricity can be prevented. However, in this configuration, a problem arises in that, since the refractive index of the film changes locally due to the conductive fine particles, the transmission factor decreases. Further, another problem arises in that incident light scatters at the film.
In addition, a conductive cover member that is disposed on the surface of a solid-state image sensor is discussed in Japanese Patent Application Laid-Open No. 2002-281362. In this configuration, the cover member disposed on the surface of the solid-state image sensor is in contact with the ground potential of a camera to thereby lower the potential of the solid-state image sensor, so that adhesion of foreign matter onto the solid-state image sensor by static electricity can be prevented. However, even in this configuration, if the focal plane shutter has a potential, a potential difference occurs between the solid-state image sensor and the focal plane shutter. Thus, an electric field occurs on the front surface of the solid-state image sensor. Therefore, a problem arises in that the electric field causes foreign matter to be attracted to the surface of the solid-state image sensor and to adhere thereto.
In addition, transparent electrodes that are disposed on the surface of a solid-state image sensor and the surface of an optical element are discussed in Japanese Patent Application Laid-Open No. 2000-29132. In this configuration, a voltage is applied to the transparent electrodes disposed on the surface of the solid-state image sensor and the surface of the optical element to thereby neutralize the electric charges occurring on the solid-state image sensor and the optical element into the same electrical potential. However, a problem arises in that the arrangement of the transparent electrodes decreases the amount of light transmission, and power consumption is required for applying a voltage.